glUseProgram installs the program object specified by program as part of current rendering state. One or more executables are created in a program object by successfully attaching shader objects to it with glAttachShader(), successfully compiling the shader objects with glCompileShader(), and successfully linking the program object with glLinkProgram().
A program object will contain an executable that will run on the vertex processor if it contains one or more shader objects of type GL_VERTEX_SHADER that have been successfully compiled and linked. A program object will contain an executable that will run on the geometry processor if it contains one or more shader objects of type GL_GEOMETRY_SHADER that have been successfully compiled and linked. Similarly, a program object will contain an executable that will run on the fragment processor if it contains one or more shader objects of type GL_FRAGMENT_SHADER that have been successfully compiled and linked.
While a program object is in use, applications are free to modify attached shader objects, compile attached shader objects, attach additional shader objects, and detach or delete shader objects. None of these operations will affect the executables that are part of the current state. However, relinking the program object that is currently in use will install the program object as part of the current rendering state if the link operation was successful (see glLinkProgram() ). If the program object currently in use is relinked unsuccessfully, its link status will be set to GL_FALSE, but the executables and associated state will remain part of the current state until a subsequent call to glUseProgram removes it from use. After it is removed from use, it cannot be made part of current state until it has been successfully relinked.
If program is zero, then the current rendering state refers to an invalid program object and the results of shader execution are undefined. However, this is not an error.
If program does not contain shader objects of type GL_FRAGMENT_SHADER, an executable will be installed on the vertex, and possibly geometry processors, but the results of fragment shader execution will be undefined.
Like buffer and texture objects, the name space for program objects may be shared across a set of contexts, as long as the server sides of the contexts share the same address space. If the name space is shared across contexts, any attached objects and the data associated with those attached objects are shared as well.
GL_INVALID_VALUE is generated if program is neither 0 nor a value generated by OpenGL.
GL_INVALID_OPERATION is generated if program is not a program object.
GL_INVALID_OPERATION is generated if program could not be made part of current state.
glGet() with the argument GL_CURRENT_PROGRAM
glGetActiveAttrib() with a valid program object and the index of an active attribute variable
glGetActiveUniform() with a valid program object and the index of an active uniform variable
glGetAttachedShaders() with a valid program object
glGetAttribLocation() with a valid program object and the name of an attribute variable
glGetProgram() with a valid program object and the parameter to be queried
glGetProgramInfoLog() with a valid program object
glGetUniform() with a valid program object and the location of a uniform variable
glGetUniformLocation() with a valid program object and the name of a uniform variable
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